Simplify rules when vector/fts used as filter

The filter_query method in Scanner allows for an FTS or vector search to be used as a filter. Currently there are branches for both prefilter and postfilter and in the FTS filter path there are branches for "match query" vs. "not match query". I find these confusing and inconsistent.

• If we perform a full text search with a vector pre-filter, and it is a match query, then we rerank the vector search results, removing rows where the score is 0.
• If we perform a full text search with a vector pre-filter, and it is not a match query, then we perform both an FTS search and a vector search and do an inner join on the results on _rowid.
• If we perform a vector search with an FTS pre-filter, then we do a flat KNN on the FTS results
• If we perform a full text search with a vector post-filter, then we rerank the results by KNN distance (and don't actually filter anything)
• If we perform a vector search with an FTS post-filter, then we remove all results that do not share a token with the query

It is confusing that a full text search with a vector prefilter becomes a vector search (and vice versa). It also seems like a vector search with an FTS prefilter is the same thing as an FTS search with a vector postfilter (and vice versa).

I propose we simplify as follows:

• Allow FTS and vector to be used as post-filters for any kind of query (even scans that are not a search)
• Define an FTS post-filter as a filter that reranks the output and (optionally) removes all rows where the BM25 score is below some threshold
• Define a vector search post-filter as a filter that reranks the output and (optionally) removes all rows where the vector search distance is above some threshold
• Do not allow FTS or vector to be used as prefilters
• Do not have different behavior for match query and non-match queries. If we can't support non-match queries in filter mode then just return a "not supported" error until we can.

Start by reading the Scanner.filter_query method and tracing its existing FTS and vector prefilter and post-filter branches. Compare the current match-query and non-match behavior with the proposed unified post-filter rules. Done means the rules are simplified as described, unsupported non-match filter cases return a clear error, and the relevant behavior is covered by tests.